[Hallicrafters] VTVM for S38 restoration

[Roger D Johnson]

Mark Shaum wrote:

> 
> For several reasons, I tend use my self-contained Simpson 260 VOM a  lot 
> more than I do the ac-powered VTVM's. It's usually handier.  I do mind 
> the fact that the VOM loads down high impedance circuitry and may 
> display an incorrect value.  A VTVM or a high input impedance 
> "transistorized" VOM is the better choice for things like AGC bus 
> measurements.

A trap for the unwary exists here! Many of the old service manuals
use voltage readings taken with a 20,000 Ohm/Volt meter and some
even with a 1000 Ohm/Volt meter. In higher impedance circuits, the
voltages can be much higher than the service literature indicates
when using a modern VTVM! This recently happened to a chap trouble-
shooting the bias string in an old radio. He was getting readings
much higher in parts of the circuit than were listed on the schematic.
Nothing wrong....the voltages on the schematic were taken with a 1000
Ohms/Volt meter which loaded down the higher impedance portions of the
circuit. The VTVM showed much higher voltages and was quite confusing
to the poor guy trying to figure out what was going on!

73, Roger


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